December 8, 2004

Common pesticides may be cause of frog deaths

Full Study

New research indicates that frequently used pesticides, including types that were once thought to be relatively benign, make be linked to the widespread disappearance of California frog populations. A researcher at California State University, Sacramento has found evidence that frog declines are associated with upwind pesticide use.

Sacramento State environmental studies professor Carlos Davidson says there is a strong association between upwind pesticide use and declines in four frog species: the red-legged frog, the mountain yellow-legged frog, the foothill yellow-legged frog and the Cascades frog. And the declines were most strongly associated with the use of cholinesterase-inhibiting pesticides, which include many of today’s most heavily used pesticides. Davidson’s findings appear in the December issue of the Journal of Ecological Applications.

The study builds on previously published findings suggesting historic applications of wind-borne agrochemicals as a factor in the declines.

“If it turns out organophosphate and carbamate pesticides are the cause, it will be a cautionary tale about the use of new chemicals,” Davidson says. “When DDT and other organochlorine pesticides were phased out because they were harming wildlife species like birds, farmers switched to a new generation of pesticide. Now, 30 years later, we’re finding those pesticides may be harming another wildlife group. It shows it may take a long time to figure out the environmental impact of new chemicals.”

Davidson mapped out the disappearance of five species of California frogs and examined the association between the spatial patterns of decline and the historic pattern of pesticide use in California from 1974 to 1991. Pesticide application data came from the Department of Pesticide Regulation records.

Davidson’s study included more than a thousand historic frog sites covering almost the entire state. At some sites, frogs were still present while at others the frogs have now disappeared. At each site, Davidson calculated the predominant wind direction and the amount of pesticide use upwind from the sites. Across the five species upwind pesticide use at sites where frogs had disappeared ranged from two to 12 times greater than for sites where the frogs still exist. Of the five species studied, only the Yosemite toad did not show declines associated with pesticide use.

The study also points to the need for further research because although Davidson found evidence that pesticides are contributing to amphibian population declines, the likely pesticide levels at most sites are well below concentrations found to be lethal in the laboratory. He says this suggests that pesticides are having sub-lethal effects and interacting with other factors.

Davidson can be reached at (916) 278-6063. Media assistance is available by calling the Sacramento State public affairs office at (916) 278-6156.

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